With development of display technology, application of active matrix organic light emitting diode (AMOLED) panel becomes more and more important. A light emitting device of AMOLED is an organic light emitting diode (OLED).
Driving schemes of the OLED pixel circuit may be classified into a voltage driving scheme (voltage type) and a current driving scheme (current type). For the voltage type AMOLED, if threshold voltages VTH of driving transistors between different pixel units are different, there is difference between driving currents IOLED driving the OLED to emit light between different pixel units. If the threshold voltage VTH of the driving transistor of the pixel drifts over time, the driving current IOLED that drives the OLED to emit light may change, resulting in afterimage. The driving current IOLED may also be different due to difference in operating voltage between OLEDs caused by non-uniformity of the OLED devices.
To the current type AMOLED, a driving current IDATA is directly supplied from external, and a voltage across a storage capacitor is determined, and thus the driving current IOLED that drives the OLED to emit light is generated. In the conventional current type AMOLED pixel structure, IOLED equals to IDATA, while IOLED must fall within a range of operating current of the OLED and is a small current. Accordingly, IDATA is also small, charging speed is low due to large capacitance of the storage capacitor, and charging time is especially very long in the condition of low gray level, resulting in not suitable for AMOLED display in high resolution and high refresh frequency.